During the past several years, significant advances have been made in improving semiconductor materials and device quality for use in long wavelength communications systems and networks. As a consequence, a new generation of optoelectronic systems and applications is expected to emerge. A precursor to this event is the development of platform integration technology devices using common and simple epitaxial materials, structures and fabrication processes. One such integrated technology platform is Photonic Integrated Circuits ("PIC"), which are often employed for transmitter purposes. PICs generally consist of combinations of active devices and passive devices, such as, lasers and semiconductor optical amplifiers coupled with modulators, couplers and waveguides.
PICs have the potential for producing low cost, high performance, advanced devices for optical communication. An important building block for PICs is a laser coupled to a passive waveguide. This fundamental structure is used for the monolithic integration of lasers with modulators, splitters, and efficient couplers to optical fibers. As known in the prior art, a laser can be integrated with a passive waveguide using a number of different techniques, including regrowth with separate optimization of the two devices, single growth on an etched substrate, disordering of multiple quantum wells ("MQW") to locally change the bandgap, selective area growth enhancement using organometallic vapor phase epitaxy, or evanescent optical field coupling in a vertical twin-waveguide ("TG") structure. Each of these coupling techniques suffer one or more of the following disadvantages: expense, complicated alignment procedures, unacceptable coupling losses or reduced reliability and performance. In particular, the regrowth technology is extremely difficult to control and has numerous problems which must be addressed. For example, one particular concern are the irregular interfaces at the regrown interfaces which can lead to poor device performance and high optical losses between adjoining devices.
Despite the above disadvantages, the vertical TG structure is useful as a versatile platform technology for PICs, since a single epitaxial growth step produces a structure on which a variety of active and passive devices can be fabricated. For example, a prior art TG laser was disclosed consisting of vertically integrated active and passive waveguide layers, which were phase-matched in a manner similar to a directional coupler. Within the past year, another prior art device was fabricated which had a GaAs/AlGaAs-based TG laser integrated with a Y-branch single mode ridge waveguide. A drawback of this latter implementation is the relative inability to control the lasing threshold current and coupling to the passive waveguide resulting from the sensitivity to variations in the device structure itself. The sensitivity variations arise from the interaction between the even and odd modes in the conventional TG structure. Due to this interaction, the fraction of the total optical power incident on the etched facet of the active waveguide is a periodic function of cavity length, which makes it relatively difficult to control the amount of power reflected from the etched facet. Alternatively stated, the interaction of the odd and even modes results in constructive and destructive interference in the laser cavity, which effects a threshold current, modal gain, coupling efficiency and output coupling parameters of the device. This interaction results in devices which are unreliable, unpredictable and expensive to manufacture. Furthermore, the modal gain and coupling length both vary dramatically with changes in the composition and thickness of each layer comprising the device.
Accordingly, there is a serious need to provide a simple and effective device which reduces the sensitivity of the device performance characteristics to laser cavity length, odd/even mode interaction and to the variations in the layered structure. This reliable and easily fabricated device can then be utilized in realizing a highly versatile, platform technology where a variety of active and passive devices can be monolithically fabricated on a single epitaxial wafer structure.